As long as you need three Shard colors for Prismatic Gems, Karnak's Temple (20 Energy Dungeon) is the most efficient place to Farm averaging 1 Tier 4 Shard per 2 Energy, but dropping random colors.

Once you are Farming for your last Prismatic Gem, you will eventually need only two colors. If one of these needed colors is Blue, you could keep Farming Karnak's Temple and use the extra Red or Yellow Shards for Lockbox. If you do not want to, then stop Farming Karnak's Temple and Farm only the colors you need at Emperor's Court (Blue), Hall of the Goblin King (Red) and Vile Gate (Yellow). Since you only want two of the three colors dropped by Karnak's Temple, the efficiency has dropped to 1 desired Tier 4 Shard per 3 Energy. Emperor's Court (Blue) averages 1 Tier 4 Blue Shard per 2 Energy, a better deal for Blue Shards.

Because you need all three colors for Prismatic Gems, do not start Farming Emperor's Court for Blue Shards. Karnak's Temple drops random colors, so you do not know how many Blue Shards you will get, and once you get all the Red or Yellow you need from Karnak's Temple, Farming the remaining Blue Shards for Emperor's Court is faster and easier.

If you are unlucky, and finish Farming all your Blue Shards from Karnak's Temple, then keep Farming it for Yellow and Red (1 desired color Shard per 3 Energy) until you are down to a single color Shard. Then switch to Hall of the Goblin King (Red) or Vile Gate (Yellow) at an average of 1 Shard per 4 Energy.